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Thresholding

Thresholding is used after pre-processing of digital images to group pixels
and thus accentuate particular elements of the image. Thresholding algorithms
are based on individual pixels (in contrast to edge or texture based methods
for example ).

The threshold can refer to different areas of the image: A global threshold
uses the entire image for calculating the value while for a local threshold
various image sections are defined and for each of them the calculation is
executed individually. As the global threshold is very sensitive to changes of
luminance it is mainly suitable for recordings with high contrasts like print
samples or transmitted light images.

Furthermore there are different kinds of thresholds: fixed and dynamic
thresholds. The latter should be used in situations with uneven illumination.

Thresholding has several steps: First the threshold is defined (e.g. by
analysing the histogram). Next the grey value of each pixel is analysed and
the pixels are grouped into segments and marked. In a simple process with only
one threshold the pixels can, for example, be represented in white if they are
above the threshold and in black if they are below the threshold, i.e. a
binarization takes place. However it is possible to define several thresholds.

Based on analysis and marking it is easy to recognize which pixels belong to
the object and which to the background.